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    //基本类型 number string boolean null undefined
    //引用类型：object （Array Math RegEx）
    
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    var str = 'hello';
    console.log(typeof str);
    var flag = true;
    console.log(typeof flag);
    var arr = [];
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    console.log(Object.prototype.toString.call([]));
    console.log(Object.prototype.toString.call({}));
    console.log(Object.prototype.toString.call(new Date));
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